Interpretive Summary: In the 1980's, a sheep producer in Oklahoma recognized that some progeny of an individual ram were very heavily muscled. Since then, it has been determined that this heavily muscled condition, called "callipyge," is caused by a single gene. The callipyge condition greatly improves carcass composition and, thus, might offer the sheep industry a method to improve retail cut leanness. However, we have shown that the callipyge condition decreases loin muscle tenderness. This experiment indicated that the callipyge condition decreases tenderness and juiciness of most major lamb muscles. Additionally, we found that the method of cookery had a profound effect on the tenderness of callipyge muscles such that callipyge roasts had similar tenderness scores as roasts from normal lambs. However, callipyge muscles were less juicy than normal muscles, regardless of cooking method. Thus, even if roasting or another tenderizing treatment were used to overcome the tenderness concerns with callipyge, palatability problems may still exist.

Technical Abstract:
Three experiments were conducted to determine the effects of the callipyge phenotype on the tenderness of several major lamb muscles and to determine the effect of method of cookery on the tenderness of callipyge lamb at 7 d postmortem. In Exp. 1, chops from normal (n = 23) and callipyge (n = 16) carcasses were open-hearth-broiled. Shear force of longissimus, gluteus medius, semimembranosus, biceps femoris, semitendinosus, adductor, and quadriceps femoris was increased 123, 44, 28, 26, 19, 16, and 13%, respectively, by callipyge (P < .05). In Exp. 2, muscles from normal (n = 18) and callipyge (n = 18) carcasses were oven-roasted. Shear force of triceps brachii was increased 11% by callipyge (P < .001); however, phenotype did not affect shear force of supraspinatus (P = .87) or psoas major (P = .64). In Exp. 3, trained sensory panel evaluation was conducted for leg roasts and open-hearth- broiled leg chops from normal (n = 60) and callipyge lamb carcasses (n = 60). Callipyge chops were less tender than normal chops (P < .05). Roasting improved tenderness of normal and callipyge muscles (P < .05); however, the effect of method of cookery on tenderness scores was greater for callipyge muscles than normal muscles. Callipyge roasts had similar tenderness as normal roasts (P = .58), and callipyge roasts were more tender than normal chops (P < .05). Regardless of cooking method, callipyge samples were less juicy than normal samples (P < .05). These data demonstrate that the callipyge phenotype will likely reduce consumer satisfaction due to reduced tenderness and juiciness; however, reduced tenderness in callipyge leg muscles could be prevented by oven-roasting.